1. Technical Field
The present invention relates to amplifiers and more particularly to an amplifier for use with a telephone transmitter mounted in a hand held telephone unit.
2. Background Art
In recent years, many new high quality but low cost telephones have been developed for the business and residential telephone subscriber. Quite frequently these units employ telephone transmitters of low output that require amplification of voice signals picked up by the telephone transmitter. Often these telephone transmitters are of the electret type with an associated built in amplifier, preamplifier or buffer amplifier included in the handset or hand held telephone unit in which the telephone transmitter is mounted. The associated receiver also included in the same telephone handset or unit is usually a standard magnetic receiver and may or may not be driven by an associated amplifier. In such telephones the voice network may be located in the handset or hand held unit or may be located in the associated telephone base. This network may be of the coupled coil type or of the integrated circuit variety.
Previous amplifier designs for use in such telephones did not take into account the hazard of feedback present under certain sound reflective conditions, where output from the handset receiver is picked up by the microphone and reinforces the output of the receiver because of the normal sidetone available in the voice network. Such "singing" is defined as an oscillation that occurs usually in the frequency range of from 2,000 to 3,000 Hz and is audible without holding the telephone receiver close to the subscriber's ear. This condition typically occurs only under extreme conditions when the handset is placed downward or in a reflective corner environment forming a type of resonant cavity around the microphone and the receiver.
Most previous designs were found to have this loud and objectionable "singing" under certain conditions, which could be transmitted over the telephone line to the other party. Early attempts to solve this problem resulted in a reduction in the transmit gain. Obviously, such signal reduction is not desirable particularly if the transmitter is to be used in anything other than short loop environments. One method used to stop the singing, involved using a feedback loop from the output of the telephone transmitter amplifier back to its input. This particular technique resulted in substantial loss in transmit level.